spray over
If you do the proper prep work and have a good base (no flaking, surface rust, & etc.) to work with, you shouldn't have any problems.
1) Scrub (and I mean scrub because you are going to a paint booth not a car show) with water that contains a good squirt of dishwashing soap (I like Dawn because it does a great job on grease). Rinse with plain water and let dry.
2) Degrease with a grease/silicone remover sold by auto paint stores. Best way to do it, is to spray it on and wipe off with paper towels. Don't rub it around, just spray and wipe.
3) Scuff with 400 (600 for metallics). Red Scrotch-Brite pads are great for those difficult areas.
4) Spray on a coat of epoxy sealer/primer.
5) Apply your favorite finish.


